A study of the simuliid fauna and their mermithid parasites in the Stony Brook watershed of New Jersey was initiated in March, 1972. Data were compiled from 9 separate sampling stations which included 5 tributaries as well as portions of the main brook. Six species, Prosimulium magnum, P. fuscum/mixtum, Simulium aureum, S. tuberosum and Cnephia mutata were parasitized by the mermithid Neomesomermis flumenalis. Seven associated species were present but not infected. Two distinct host-parasite cycles were evident. Members of the genus Prosimulium maintained a winter cycle at all 9 sites which extended for the duration of the larval period, i.e. late November through May. A summer cycle, which lasted from early May through October was maintained by S. tuberosum but only at 1 of the 9 sites. Other stations had high S. tuberosum populations but no mermithid infestation. Constant water flow appeared to be responsible for this variation; data indicated that water management might enhance mermithid parasitism. At all sites, parasitism of blackfly larvae occurred within 3 weeks of hatching; infection of later instars did not occur. Data indicated that release of cultured pre-parasitic stages would only be effective immediately after a blackfly hatch, i.e. less than 3 weeks out of an entire year with univoltine species.